Many modern day electronic devices include optical imaging devices (e.g., digital cameras) that use image sensors. An image sensor may be disposed on an integrated circuit (IC) which includes an array of photodetectors and supporting logic. The photodetectors, which can correspond to individual pixels, measure incident radiation (e.g., light) corresponding to an optical image, and the supporting logic facilitates readout of digital data from the IC. The digital data output from the IC corresponds to a digitally encoded representation of the optical image.
Standard IC manufacturing processes can produce image sensors that use frontside illumination (FSI) techniques or backside illumination (BSI) techniques. With FSI, light falls on a frontside of the IC, and passes through an electrical interconnect structure, such as a stack of back end of line (BEOL) metal layers, before being collected at the photodetectors. Often in FSI, the BEOL metal layers are structured to have openings (apertures) over the individual photodetectors, as the material of the BEOL metal layers can otherwise block light if arranged between the incident light and the photodetectors. To optimize the amount of light that reaches the photodetectors through these apertures; microlenses, waveguides, and other optical features are often used in FSI to minimize reflections and help direct light towards the respective photodetectors.
In BSI, rather than having light pass through openings/apertures in the BEOL metal layers, the sensor is illuminated from the backside (i.e., the face opposite the stack of BEOL metal layers). Compared to FSI, BSI allows a photodetector to have its electrical components on one face of the sensor and its optical path on the other, which allows better separation of optical elements from electrical elements. This means the optical path can be optimized independent of the electrical components and vice versa. The optical constraints for BSI are similar to FSI, except in BSI the photodetectors are often positioned closer to the microlenses, which are now disposed on a thinned down substrate surface. Also, because BSI removes the constraints associated with apertures in the BEOL metal layers, BSI eliminates a loss mechanism for incident light, potentially providing a higher quantum efficiency for the devices.
FSI and BSI technologies are both valuable market segments, with FSI being an established technology that is favorable in lower-cost applications with larger pixels, and BSI being an emerging technology that is favorable in higher-end applications with smaller pixels.